3.4 Reacting quantities

Question 1

What is stoichometry

Answer 1

The ratio of moles of substances in a balanced equation

Question 2

What do chemists use balanced equations for

Answer 2

To find:
The quantities of reactants required to prepare a required quantity of a product
The quantities of products that should be formed from certain quantities of reactants

Question 3

Method to identify an unknown metal

Answer 3

1. Set up apparatus - Flask and a gas syringe
2. Weigh a sample of the metal and add it to the flask
3. Using a measuring cylinder, add 25.0cm^3 1.0mol dm^-3 HCl to the flask and quickly replace the bung
4. Measure the maximum volume of gas in the syringe

Question 4

What is the theoretical yield?

Answer 4

The maximum possible amount of product

Question 5

Why is the theoretical yield difficult to achieve

Answer 5

The reaction may not have gone to completion
Other reactions (side reactions) may have occurred alongside the main reaction
Purification of the product may result in loss of some product

Question 6

What is the actual yield?

Answer 6

Usually lower than the theoretical yield

The actual amount of product formed

Question 7

Equation for percentage yield

Answer 7

Percentage yield = 100(actual yield/theoretical yield)

Question 8

What is a limiting reagent

Answer 8

The reactant that is not in excess and will be completely used up first, stopping the readtion

Question 9

How to find which is in excess from equations

Answer 9

Eg. 2H2+O2

If the same amount of both substances are allowed to react, hydrogen will be used up first and half of the oxygen will remain unreacted, therefore the limiting reagent is hydrogen. Then calculations must be based on hydrogen

Question 10

What is atom economy

Answer 10

A measure of how well atoms have been utilised

Question 11

Reactions with high atom economies…

Answer 11

…produce a large proportion of desired products and few unwanted waste products, and are important for sustainability as they make best use of natural resources

Question 12

Equation for atom economy

Answer 12

Atom economy = 100(sum of molar masses of desired products/sum of molar masses of all products)

Question 13

How to decide if something is sustainable

Answer 13

Look at atom economy
Look at how readily available reactants are
Look at efficiency and percentage yields